ON THK CHANGE OF REFRANGIBILITY OF LIGHT. 383 A very marked internal dispersion was produced, but the nature of the effect depended in a good measure on the character of the spark. A feeble branched spark, giving but little light, and making little noise, produced an illumination extending to a considerable depth, and very much stronger than that occasioned in the same solution by the flame of a spirit-lamp. The rays by which this was produced passed in a great measure through a plate of glass interposed between the spark and the surface of the fluid. But a bright linear spark, making a sharp crack, produced an illumination almost confined to an excessively thin stratum adjacent to the surface of the fluid; and the rays by which this was produced were cut off by glass, though transmitted through quart/,. The same was the case with the discharge from a Leyden jar, which produced a bright light almost confined to the surface*. 218. The opacity of a solution of sulphate of quinine appears to increase) regularly and rapidly with the refrangibility of the rays incident upon it. Hence we are led to the conclusion that a st.rong electric spark is excessively rich in invisible rays of extremely high refrangibility. Glass is opaque with respect to these rays, but quartx; transparent. It is known that the phosphorogenic rays of an electric spark, at least those which affect Canton's phosphorus, pass very freely through quart/, but are stopped by a very moderate thickness of glass. This alone, after what has been already mentioned, would lead us to suppose that the phosphorogenic rays coming from such a spark arc merely rays of very high refrangibility. If so, they ought to be intercepted by a very small quantity of a substance, known to absorb such rays with energy. After having made some experiments on the production of phosphorescence iu Canton's phosphorus by means of an electric discharge, and observed how the influence of the discharge was transmitted through quartz and stopped, or almost entirely stopped, by glass, I felt confident that my own observations were comparable with those of others. A small portion of the phosphorus was then placed on card, covered by an empty quartz vessel, and had the discharge of a Leyden jar passed over it. The phos- * See note J.